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Preliminary Design and Testing of a VTOL Light UAV Based on a Box- Wing Configuration

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Preliminary Design and Testing of a VTOL Light UAV Based on a Box- Wing Configuration Preliminary Design and Testing of a VTOL Light UAV Based on a Box- wing Configuration Giuseppe Palaia and Vittorio Cipolla University of Pisa Department of Civil and Industrial Engineering - Aerospace Section, Pisa, Via G. Caruso 8, 56122 ITALY [email protected] and [email protected] Vincenzo Binante and Emanuele Rizzo Skybox Engineering - spin-off company of University of Pisa Via G. Caruso 8, 56122 ITALY [email protected] and [email protected] Keywords: Aircraft Design, Box-Wing, VTOL, UAV, Prandtl-Plane. ABSTRACT Since early 1950s convertiplanes have been studied for approaching to Vertical Take-Off Landing, but a lot of concern was about safety. Nowadays, these architectures are also starting to catch on in civil application thanks not only to the development of more reliable flight control systems but also to the possibility of integrating aircraft disruptive configurations. This paper aims to present a preliminary study on unconventional configuration based on the Best Wing System concept by L. Prandtl. The developed Unmanned Air Vehicle is called “TiltOne” because it can tilt its wings by 90 degrees, switching from multicopter configuration to box-wing configuration. The preliminary design has been addressed from several points of view: a conceptual design has been carried out thanks to in-house optimization tool; aerodynamic performances, propulsion design, and mechanical design have been addressed in order to make the prototype for preliminary vertical flight test. The aerodynamic configuration is helpful for endurance and payload performance; however, particular attention must be paid to the propeller choice to reduce current consumption during the mission. 1 INTRODUCTION Convertiplane are vehicles initially developed in 1950s and 60s to investigate unconventional configurations capable of Vertical Take Off and Landing (VTOL). The first developed prototypes were the Hiller X-18 (tiltwing), the Vertol VZ-2A (tiltwing) and the Curtiss-Wright X-19A (tiltrotor), ancestor of the more “recent” Canadiar CL-84 and Bell-Boeing V-22 Osprey. The development of these projects, mainly under experimental military programs, highlighted how the conversion can be very critical, so the typical conversion corridor has to be deeply investigated. Nowadays, these architectures are catching on in civil application, but a deep study has to be done for the development of more reliable flight control system and the integration of disruptive aircraft configurations. For example, Airbus [VI] and Uber [VII] are developing VTOL aircraft for the conception of Urban Air STO-MP-AVT-323 27- 1 Preliminary Design and Testing of a VTOL Light UAV Based on a Box-wing Configuration Mobility (UAM). Nevertheless, the transition phase (from multicopter to fixed-wing configuration and vice- versa) is a hard challenge and a lot of control techniques have been developed to overcome the problem [12]. A promising disruptive configuration is the so-called “box-wing” which is based on the Best Wing System concept by L. Prandtl. This configuration, which in honour of Prandtl researchers at University of Pisa called “PrandtlPlane” ([4],[5]), shows several advantages: high efficiency, smooth post-stall behaviour, good damping in pitch dynamic, good structural stiffness, and the possibility to allocate surface controls in a different way (e.g., two counter-rotating elevator can be placed on the two wings in order to obtain a pure pitch control). For the previous reasons this configuration is very interesting for both military and civil [VIII] transportation. The Unmanned Air Vehichle (UAV) presented in this work has been called “TiltOne” because of the capability of tilting its wings by 90 degrees and switching from multicopter to fixed-wing configuration. It is a tilt-wing, with PrandtlPlane configuration in forward flight, capable of taking off and landing vertically. This study focuses on the preliminary design of the UAV which involves several design areas. The conceptual design has been addressed with an in-house optimization code taking into account aerodynamic, propulsion, and flight mechanic. By the evaluation of the aerodynamic performance it was possible to define the constraints for the propulsion system. A proper mechanical design has been done in order to define the main components for the manufacturing. Thanks to its modularity, a preliminary flight test has been completed (without wings) defining the main parameters of the flight controller in multicopter configuration. 2 LAYOUT OF THE TILTONE The TiltOne. is a tilt-wing, with four mounted propellers, capable of vertical take-off, vertical landing, and forward flight tilting both wings and propellers around the wing axes. Its configuration is relatively different respect to other convertiplanes (e.g. SUAVI [1] or QTW of Chiba University [11]); in fact it has two tilting wings (one forward and one backward) mounted on different heights: the forward wing is on the same level of the fuselage whereas the rear one is higher, so there is a vertical gap (h) between the two wings. Generally, for a box-wing configuration, the parameter indicating the vertical gap is the ratio h/b (where b is the span); the induced drag reduction is strongly dependent by the vertical gap and the lift distribution, as shown by Prandtl [13]. With a symmetrical lift distribution is possible to obtain the so-called “Best wing system”, the configuration with the lowest induced drag and the maximum lift-to-drag ratio (also referred as aerodynamic efficiency). This configuration can allow different advantages verified in previous works ([6], [7] and [8]), as: • A smooth post-stall which promises a better aircraft behaviour near the transition phase. • A higher aerodynamic efficiency hence an increase of flight endurance or payload capabilities. • An enhancement of structural stiffness. • A damped dynamic pitch due to the wing position along the longitudinal axis [14]. The PrandtlPlane, as shown in previous works ([6],[7]), is also capable of Conventional Take-Off and Landing (CTOL). We believe that this configuration can have some advantage in the transition phase for the smooth post-stall behaviour, and it can be adopted for improving range or payload capability (for the higher aerodynamic efficiency). Thanks to its features, it seems reasonable to assume that the PrandtlPlane can be used in both military and civil application, in particular for improving safety in critical military operations. 27- 2 STO-MP-AVT-323 Preliminary Design and Testing of a VTOL Light UAV Based on a Box-wing Configuration 3 CONCEPTUAL DESIGN DROPT (DRone OPtimization) is an in-house optimization tool developed to select the best parameter combination for the design of an aircraft with box-wing configuration. DROPT can be applied to the design of different aircraft categories: from small UAV to general aviation. The optimization runs one variable at time, so it is possible to choose for each simulation one of the following objective functions: • Maximizing the endurance for a given mission profile. • Minimizing the energy consumption in the mission. • Maximizing the cruise speed. • Maximizing the payload. The optimization problem is complete if the constraints equations and the range of optimization variables are defined. Several non-linear constraints equations have been defined (e.g. geometry, maximum current absorbed by the motors, and minimum available thrust); it implies that a proper algorithm has to be selected in order to solve the problem, so a Genetic or Non-Liner Programming (NLP) algorithm can be selected. The optimization variables for this problem are defined as follow: number of motors or propellers (N), propeller diameter (D), propeller advance ratio (J), propeller static thrustcoefficient (CT0), wingspan (b), wing chord (c), motor speed in hovering and cruise (n0 and n), and the angle of attack (α). The TiltOne can afford both conventional and unconventional mission; it can take-off like a “fixed-wing” aircraft or like a rotorcraft. In this work an unconventional mission has been chosen. The mission profile (depicted in Figure 3-1) is divided into three parts: in the first one the UAV takes-off with a vertical speed (Vz); when the right altitude (zcruise) is reached the velocity gradually changes; the vertical speed goes to zero, in the meanwhile, the horizontal speed goes to the cruise speed (Vxcruise). In the third phase, the horizontal and vertical speeds mutually change again, and the aircraft lands. The time for take-off and landing is fixed while the cruise time is the variable to be optimized. Figure 3-1: Mission profile for the TiltOne STO-MP-AVT-323 27- 3 Preliminary Design and Testing of a VTOL Light UAV Based on a Box-wing Configuration The following optimization problem has been formulated: min− t 2 T 0/ MTOW≥ K 0 F x/ Drag= 1 F y/ MTOW = 1 RPM required≤ RPM allowed NK span D≤ 2b J required ≤ J max Pmotreq≤ Pmotormax AR≤ ARmax Irequired ≤ I max n≤ N parM l ≤ x≤ u { b b • T0/MTOW ≥ K0 defines that the static thrust T0 must be higher than the MTOW. An extra thrust of about 20% is requested. • MTOW is the maximum take-off weight. It is calculated according the following equations MTOW= W pay+W struct+W elect W elect = W bay+W props+W mot W struct= W wing +W fus W batt= E/ Espec 2 W props= 0.0005 D W = 0.0001 I 2+0.001 I if I≤ 90 A { mot Wpay is the payload weight and is about 300g. Wstruct is the structural weight and is composed by the 2 wing (Wwing) and the fuselage (Wfus): the first is proportional to the wing surface (measured in m ) while the second is fixed. Wbatt is the weight associated to battery; it depends on the total requested battery energy (E) and the specific battery energy (assumed as Espec = 185 Wh/kg). Wprops is the propeller weight depending on the propeller diameter (D measured in inch).
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